Recently, in a MIMO system, there is a tendency to increase a distance between antennas to obtain a better channel environment. In particular, conventionally, an oscillator has been shared by antenna stages of a MIMO system, as shown in FIG. 1. However, recently, due to the problem in implementing a RF circuit occurring when a superhigh frequency is used, independent oscillators have been increasingly used by the respective antenna stages as shown in FIG. 2.
Meanwhile, a phase noise refers to a noise caused by nonideal characteristics of an oscillator and is in the form of random process. Such a phase noise is independently generated from each oscillator. If independent oscillators are used by antenna stages, respectively, the number of phase noises affecting a system is increased in proportion to the number of antennas. That is, performance degradation caused by a phase noise may be increased in this system as compared to a conventional system in which a common oscillator is used by antenna stages.
Specifically, a phase noise generated from a RF stage by nonideal characteristics of an oscillator greatly affects the performance of an orthogonal frequency division multiplexing (OFDM) system. The phase noise may cause performance degradation in the form of a common phase error (CPE) and an inter-carrier interference (ICI). Particularly, the common phase error more greatly affects the performance degradation. Therefore, conventionally, studies on the estimation of a common phase error have been actively conducted.
In this regard, Korean Patent Laid-open Publication No. 10-2003-0098224 (entitled “An apparatus for phase noise suppression in the wireless LAN system, and a method thereof”) suggests a common phase error correction algorithm for the case where a phase error is present and an algorithm for reducing an inter-carrier interference. Herein, the common phase error correction algorithm is extended from a least square algorithm. However, this conventional phase noise mitigation method is limited in that it is based on the assumption that only a single antenna is used.
As for a multiple-antenna system in which each antenna stage has an independent oscillator, phase noises are generated in proportion to the number of antennas, resulting in severe performance degradation. However, an algorithm for mitigating the phase noises in this system has hardly been studied. That is, the conventional least square algorithm can estimate only a common phase error among the effects of phase noises, and if only the common phase error is compensated, performance improvement may be expected at a low SNR power but cannot be expected at a high SNR power. In this case, a bit error rate (BER) graph can also confirm that an error-floor phenomenon occurs at a high SNR.
Accordingly, there is a need for a phase noise mitigation method capable of reducing not only a common phase error but also an inter-carrier interference caused by phase noises in a MIMO-OFDM system in which each antenna stage has an independent oscillator.